Recently, attention has been paid to a semiconductor pressure sensor utilizing the piezo resistance effect of semiconductor such as silicon and germanium.
For instance, the inventors have proposed a film pressure sensor which is formed with an n-type microcrystalline silicon (.mu.c-Si) formed by a plasma CVD method as a pressure-sensitive resistance layer (Japanese Patent Application No. 111377/1986).
In the film pressure sensor, not only the adhesion between the diaphragm and the gauge section, but also the adhesion between the insulating layer and the pressure-sensitive resistance layer is satisfactory; however, the pressure-sensitive resistance layer is still unsatisfactory in temperature characteristic.
In addition, a structure using a p-type polycrystal silicon as its pressure-sensitive resistance layer has been proposed ("Sensor Technique", December 1985, Vol. 5, No. 13, pp. 30-34).
In formation of a p-type polycrystalline silicon layer by the plasma CVD method, it is necessary to use high power, because if high frequency power (RF power) less than 100 W is used, then it will become amorphous. In the case also where the gas ratio is changed, it is necessary to employ high power. However, it is well known in the art that plasma uniformity is, in general, low with high power.
Accordingly, in the case where a film pressure sensor is formed with the p-type polycrystalline silicon layer as the photo-sensitive resistance layer, the photo-sensitive resistance layer is improved in temperature characteristic; however, it is not uniform in resistance. Therefore, the film pressure sensor thus formed is disadvantageous in that in formation of a bridge circuit, it is necessary to perform resistance compensation which is rather troublesome.